Charge-transporting thin films made of organic compounds are used as light-emitting layers and charge-injecting layers in organic electroluminescent (EL) devices. In particular, a hole-injecting layer is responsible for transferring charge between an anode and a hole-transporting layer or a light-emitting layer, and thus serves an important function in achieving low-voltage driving and high brightness in organic EL devices.
Processes for forming the hole-injecting layer are broadly divided into dry processes such as vapor deposition and wet processes such as spin coating. Comparing these different processes, wet processes are better able to efficiently produce thin films having a high flatness over a large area. Hence, with the progress currently underway toward larger-area organic EL displays, there exists a desire for hole-injecting layers that can be formed by wet processes.
In view of these circumstances, the inventors have developed charge-transporting materials which can be used in various wet processes and which, when used to form hole-injecting layers for organic EL devices, are capable of achieving excellent EL device characteristics. The inventors have also developed compounds of good solubility in organic solvents for use in such charge-transporting materials (see, for example, Patent Documents 1 to 4).
However, improvements in wet process materials for hole-injection layers are constantly being demanded. In particular, there is a desire for wet process materials which give thin films having excellent charge transportability.